Block copolymers can assemble into large-scale, dense, and regular nanoscale patterns, making block copolymer assembly a potential candidate for nanolithography. The pattern of a block copolymer material can follow a chemical pattern to generate a regular structure, in a technique referred to as directed self-assemble (DSA) by chemical epitaxy. Polystyrene (PS) based chemical patterns are a commonly used system and can be fabricated by dissolving PS in solvent and casting the solution uniformly over large area.